1. Field of the Invention
The present invention relates to improved cartridge filters and materials and methods for producing them. More particularly, the present invention relates to an improved pleated filter media and cartridge filters fabricated from improved composites of polytetrafluoroethylene (PTFE) and other high temperature and chemical resistant fibers.
2. Description of Prior Art
Fiberglass textiles are widely employed today in numerous applications. This is due to the many favorable properties of fiberglass, including relatively low cost, excellent mechanical strength, dimensional stability, good insulative properties, and excellent resistance to temperature and humidity. Among the popular present uses of fiberglass are as an architectural fabric, in the electronics industry (e.g. as a printed circuit board), as a filtration media, and as a structural composite.
Due mostly to its relatively low cost, fiberglass presently dominates the high temperature filtration market (typically defined as those filters operating at temperatures greater than 300.degree. F.). In this regard, fiberglass often costs 33 to 95% less than some competitive synthetic textile fibers (e.g. polytetrafluoroethylene, aramids, polyphenylene sulfide, polyimides and copolyimides).
Although fiberglass performs adequately in these applications, it suffers from numerous deficiencies. Perhaps the greatest problem with fiberglass is that it is quite fragile, being easily damaged when flexed or abraded. As a result. fiberglass fabric is incapable of performing in many applications. For instance, where a cartridge or bag filter must be vigorously flexed to free filtrate and clean the filter media (e.g. in a "shaker"-type bag house filter or a pulse-jet bag house filter), a fiberglass fabric will quickly fail. Even under less demanding regenerative procedures (e.g. reverse air cleaned bags), where far less bag flex is encountered, fiberglass has relatively limited operational life.
Another deficiency of fiberglass is that it is subject to certain chemical attack. Chemicals attack the glass filaments in the textile primarily when the process gas phase goes through a dewpoint excursion. Chemical attack can also occur in the gaseous phase or when solid particulate contacts the fiberglass.
In an effort to address some of the problems of chemical attack, a number of finishes and protective coatings have been developed. Examples of such treatments include constituents of silicone oils, graphite and PTFE dispersion coatings. Although such treatments have proven relatively effective at protecting fiberglass from chemical attack, they do not improve the problem of flex failure.
Another common problem with fiberglass is that it is difficult to handle and work. By way of example, if the fiberglass is not carefully handled during weaving, the low abrasion resistance of the fiberglass often leads to friction damage to the glass fibrils. Likewise, abrasion caused by weaving equipment can result in surface imperfections (e.g. glass pills) which can cause problems in later processing. Further, even a single fold in the glass material during processing can lead to fatigue and later failure.
In light of these many problems, synthetic fibers are the fibers of choice where long filter life is needed and/or extreme operative conditions are expected. Unfortunately, the costs of synthetic fibers restrict their accepted uses. Additionally, certain synthetic fibers, while having many exceptional characteristics, are also limited in certain respects. For example, PTFE tends to have stability (creep) problems in reverse air bag houses, with tension on the bag elongating it to a point at which proper cleaning and flexure does not occur. Copolyimides and aramids hydrolyze in the presence of moisture and high temperatures. Polyphenylene sulfide oxidizes and embrittles when exposed to oxygen and high temperature.
U.S. Pat. Nos. 5,456,983 and 5,549,966 overcome many of the problems mentioned above with respect to performnance, particularly in the case of bag filter applications. These patents teach a composite fiber comprising a coherent strand of fiberglass and PTFE which is far more resistant to flex, abrasion, and chemical attack than previous fiberglass fibers. The composite fiber is preferably created by combining an expanded PTFE tow yarn and a fiberglass yam in an air-jet texturing apparatus. By so combining, the filaments of expanded PTFE become intertwined around the filaments of fiberglass so as to form a single strand of strong, flexible composite material. The material can be readily formed into a fabric with a wide range of possible uses, including as a filter media, as an architectural fabric, as a structural fabric (such as when combined with an epoxy resin), etc. It is taught in these patents that the demands of filtration require improved flex fatigue resistance in the filling direction only, and as such, the composite fibers need only be applied in that direction.
While these materials have experienced significant success in the case of bag filter applications, significant limitations still exist with respect to the performance of these materials in cartridge filter applications.
Thus, it is a purpose of the present invention to provide an improved heat and chemical resistant pleated composite material and improved filter cartridges which can withstand the rigors of pleated cartridge filtration filtering and cleaning operations.
It is another purpose of the present invention to provide an improved thermoset resin impregnated composite material with self supporting pleats that provides superior flex life and durability over existing high temperature and chemical resistant structural fabrics. Most current filters made from heat and chemical resistant fabrics require a metal support structure to hold the non-rigid fabric into a pleated form as taught by Manniso et al in U.S. Pat. No. 4,878.930. Pleated cartridge filters made from rigid nonwoven materials offer the benefit of greater filter area and lower weight than those with metal support structures. Cartridge filters made from composite laminates of expanded PTFE and nonwoven or felt textile backings, as taught by Tronto et al in U.S. Pat. No. 5,207,812, allow open pleats, spaced with less than 7 per inch, without requiring external support structures. The open pleats maximize cleaning effectiveness by expanding and flexing during the pulse jet cleaning process; however, current rigid, high temperature materials are too brittle to withstand repeated flexing. The present invention provides such an improved pleated structural composite and improved cartridges which can withstand this flexing without cracking or breaking, and operate in high temperature and corrosive environments.
It is a further purpose of the present invention to provide an improved pleated composite material and improved cartridges which can withstand flexing of the filter media with the pleat folds being parallel to the filling direction, whereby the greatest increase in flex life and durability of the pleated filter media in a cartridge can be gained by inclusion of composite yarns of expanded PTFE and a high temperature and chemical resistant material in the warp direction or in both the warp and filling directions.
These and other purposes of the present invention will become evident from a review of the following specification.